Field of the Invention
The invention relates to a portable communication device, such as a hearing instrument, comprising a transceiver unit for establishing a wireless communication link using a digital modulation scheme with a packet-based frequency hopping scheme and an antenna.
Description of Related Art
Antennas for integration in miniature communication systems, such as hearing aids, need to be small, efficient and have to satisfy specific requirements in terms of bandwidth. Due to physical limitations (Chu-Harrington limit), meeting all criteria at the same time is not possible, so that a small antenna can be either broadband at the expense of efficiency or more efficient if bandwidth is sacrificed.
The required bandwidth of an antenna is given by the application it is used for. Typically, an additional bandwidth margin is foreseen to compensate for detuning of the antenna by its environment which may happen, for example, due to the influence by the user interacting with the communication device.
In channel-based systems a whole frequency band needs to be covered, even if the particular communication channel requires only a fraction of the spectrum allocated to the band. An example of such an application is the Bluetooth Low Energy standard, which operates in the ISM frequency band from 2.400 to 2.484 GHz. This frequency band is divided into 40 channels, each channel having a bandwidth of only 2 MHz. Even though the size of each individual channel is rather small, a Bluetooth antenna needs to cover the whole bandwidth of at least 84 MHz, leading to a reduced efficiency compared to the case where just a single channel is covered.
Antenna efficiency directly impacts the link budget of the whole system. In particular, for battery powered devices transmit power is quite limited and cannot be arbitrarily increased. In addition, receiver sensitivity is bound to physical limits. Thus, antenna inefficiency cannot be compensated for, resulting in a reduced communication range.
A tuned antenna system may typically overcome such seemingly conflicting requirements of a small and efficient antenna by reducing antenna bandwidth to the minimum required by a single channel. Such a system adapts the antenna resonance frequency with a tuning circuit.
According to one approach, the antenna resonance frequency is adapted based on the selected channel. This approach requires that the antenna bandwidth is larger than the channel bandwidth in order to compensate for component tolerances and antenna detuning caused by the environment. Hence, this solution does not achieve the best possible antenna efficiency.
According to an alternative approach, the antenna resonance frequency is dynamically adjusted, based on the selected channel, as well as a measure of the antenna reflection coefficient. This approach allows to have an antenna with the lowest possible bandwidth and thus the highest efficiency, as well as being capable of compensating for any detuning effects caused by the antenna's environment. However, also this approach has some drawbacks. First, a directional coupler in the path between the transceiver and the antenna is required. Such a coupler introduces additional losses and can take up significant space. Second, the tuning can only be done in transmit operation. Third, a power detection circuit is required to measure the amount of reflected energy from the antenna.
U.S. Pat. No. 7,817,969 B2 relates to FM receiver system for establishing communication between devices like a mobile phone, a laptop computer, a personal computer or a PDA, wherein the FM antennas are tuned in order to alter the center frequency of the FM antenna gain profile to compensate for environmental changes; in a recovery operation mode and in a tracking operation mode the FM antenna control circuitry provides frequency hopping across the FM frequency band at a scanning frequency interval and at a dithering frequency interval which is less than the scanning frequency interval, respectively. A similar system is described in U.S. Pat. No. 7,844,242 B2.
U.S. Pat. No. 8,391,817 B2 relates to a method of tuning an antenna used in data transmission, wherein a first signal strength indicator (RSSI) indicating a signal strength of a first data signal transmitted by an antenna on a first frequency is received, a second RSSI indicating a signal strength of a second data signal transmitted by the antenna on a second frequency different to the first frequency is received, and a tuning control signal is determined based on the first RSSI and the second RSSI, with the antenna being tuned based on the control signal. The RSSI may be checked before any signal measurement is done in order to avoid measuring in the presence of interference; for example, it may be checked that RSSI is below a defined threshold.
U.S. Pat. No. 5,136,719 relates to a method of tuning a radio antenna in a wrist watch paging receiver which is periodically activated to receive packets of information. The antenna is automatically tuned on a periodic basis prior to the receipt of a packet of information. During each tuning operation, the antenna is swept across an entire tuning range to an optimum level, with a RSSI signal being monitored to detect the value of the tuning element that produces the maximum RSSI signal.
European Patent Application EP 1 782 578 A2 and corresponding International Patent Application Publication WO 2006/020520 A2 relate to a wireless communication medium in WLANs, wherein the results of physical measurements, such as the RSSI, are used for antenna calibration.
EP 2 434 652 A1 relates to an antenna which is tuned according to the value of a quality function based on the RSSI and a power indicator.
U.S. Pat. No. 8,068,800 B2 relates to a tuning method of electrically small radio receiver antennas, wherein the RSSI is used to produce a control voltage which controls a capacitance in an antenna matching circuit such that the ratio of a change in the RSSI to a change in the control voltage is minimized.
European Patent Application EP 2 552 017 A1 relates to the minimum distance problem in a wireless communication link between mobile devices. An automatic gain control (AGC) gain value is automatically determined in the receiver for application to an AGC input signal to provide an appropriate signal level of the recovered electric signal when the level of the AGC input signal is in a range between a minimum signal level and a maximum signal level corresponding to maximum and minimum AGC gain values, respectively; the impedance matching of the antenna to the receiver is changed in the impedance matching circuit based on the AGC gain value.
International Patent Application Publication WO 2012/084026 A1 relates to a system comprising a telephone at a hearing aid, wherein the analog signal to the loudspeaker of the telephone is picked up and digitalized at the telephone; the digital signal then is wirelessly transmitted to the hearing aid using the radio transceiver at the telephone, coupling with a radio at the hearing aid. Parameters of the hearing aid can be adjusted by tones sent over a telephone network to the telephone and from there to the hearing aid radio circuit; the hearing aid is adapted for tuning the electrical components of the hearing aid upon detection of a certain tone signal and for tuning an antenna of the radio circuit.
U.S. Pat. No. 9,432,780 B2 relates to wireless communication of information for a hearing assistance device including a multi-mode radio adapted to provide communications at different frequencies using frequency control. An antenna matching circuit is programmable for a selected frequency of operation of the multimode radio.